1. Field of the Invention
The invention relates to a system and a method for distributing positioning information of a positioning system, and more particularly to a system and a method which uses a data network, such as the Internet, to distribute global positioning system (GPS) satellite information to roving GPS receivers.
2. Description of the Related Art
The Global Positioning System (GPS) includes approximately twenty four satellites each orbiting the earth at a substantially constant speed and altitude. In the GPS system, each satellite sends out, at precisely synchronized times, a code sequence which identifies the satellite. Specifically, the code sequence transmitted by each satellite is a precisely timed binary pulse train including a C/A-code (Coarse Acquisition Code) for civilian use and a P-code (Precision Code) for military use. The code sequence transmitted by each GPS satellite contains a unique C/A-code identifying the satellite. The chipping rate of the C/A-code sequence is 1 million bits per second and repeats every one one-thousandth of a second.
Besides transmitting the C/A-code and the P-code sequences, the GPS satellites also transmit a 50-bit-per-second GPS navigation message data stream superimposed on the C/A-code and the P-code pulse trains. The GPS navigation message transmitted by each GPS satellite includes information defining the orbital location, clock and status of the satellite. The data stream of the GPS navigation message is divided into 30-second frames. Each frame is further divided into five 6-second subframes. The first subframe (Subframe 1) contains the satellite clock correction factor. The second and third subframes (Subframes 2 and 3) contain the orbital parameters (also called the ephemeris constants) defining the current orbit of the satellite. In the present description, the term “ephemeris information” or “ephemeris data” is used to refer to the ephemeris constants or the orbital parameters of the GPS satellites contained in the GPS navigation message. The fourth subframe (Subframe 4) of the GPS navigation message contains messages such as the satellite health status information and the ionospheric distortion in the atmosphere. The fifth subframe (Subframe 5) contains an almanac of the GPS satellite constellation.
One common application of the GPS system is position determination. The position determination result of a GPS system is superior to other positioning mechanisms, since it is not affected by weather conditions to the same extent as other positioning mechanisms. Further, since no ground stations are typically involved in GPS positioning, a navigation system based on GPS has unlimited range. In summary, GPS positioning information is available 24 hours per day at all locations worldwide.
A GPS receiver determines its position by first finding the GPS satellites that are above the horizon at the moment. Then, the receiver acquires the code sequences and the ephemeris information from four or more GPS satellites in view. To locate the satellites that are above the horizon, a GPS receiver, having no knowledge of the GPS satellite constellation, typically has to search through a predefined range of frequencies to tune into the GPS satellite signal. Because of Doppler effect, the satellite signal may not appear exactly at the two predefined L-band frequencies assigned for GPS satellites. The GPS receiver has to search through a frequency range to tune into the GPS signals. Simultaneously, the GPS receiver must search through all time shifts of the 24 C/A-codes to find the C/A-code that matches those contained in the received signal. In this manner, the GPS receiver finds and identifies the four or more satellites in view. Of course, a GPS receiver may have in memory a last known position and last known GPS almanac information based on a previous position determination. In that case, the GPS receiver can use the stored information as estimates to locate the satellites most likely to be above the horizon at the moment. The process of locating the GPS satellites in view can be time-consuming, particularly when the receiver does not have prior, useful positional or GPS almanac information.
After finding the GPS satellites in view, the GPS receiver proceeds to acquire the time of arrival information and the ephemeris information from four or more GPS satellites. The time of arrival information is obtained by correlating a replica of the expected code sequences with the received code sequences. Typically, a binary pulse train from a GPS satellite takes about one-eleventh second to arrive at a receiver on the ground. Using the time of arrival information, the GPS receiver computes the signal travel times and the pseudo-range information to each satellite. The GPS receiver uses a trilateration technique to obtain a “measured” position of the receiver. The measured position typically refers to the three-dimensional position coordinates including the longitude, the latitude and the altitude of the receiver. In some situations, only the two-dimensional position coordinates are of interest and in those cases, a GPS receiver only needs to acquire GPS signals from three GPS satellites for position determination. To perform trilateration, the GPS receiver operates on the pseudo-range measurements based on the acquired code sequences from four or more GPS satellites and the ephemeris information of the same four or more satellites at the time the acquired code sequences were transmitted. Because the ephemeris information are contained in two subframes of the GPS navigation message data stream, an acquisition time of at least 12.5 seconds is needed for a GPS receiver to acquire the necessary ephemeris information for the GPS satellites. Thus, a GPS receiver must be remain in clear line-of-sight of the GPS satellites for at least 12.5 seconds to enable the receiver to acquire the necessary ephemeris information. In practice, the GPS receiver needs to observe the GPS satellites for an even longer period of time because the GPS receiver may need other satellite information contained in the navigation message. Because the navigation message is updated every 30 seconds, an observation period of at least 30 seconds is needed for the GPS receiver to acquire the entire navigation message.
Although this lengthy acquisition time poses no problem for GPS receivers mounted on aircraft or used in geological or archaeological expeditions where the receivers remain mostly exposed to the open sky, the 12.5-second acquisition time for ephemeris information can become a problem for GPS receivers mounted on roving mobile units traveling in an urban environment, particularly where the area is densely built. Presently, typical GPS applications include GPS receivers mounted in vehicles or contained in cellular telephones. In these applications, the GPS receivers can only observe the GPS satellites intermittently. These roving GPS receivers typically cannot maintain contact with any GPS satellite for the lengthy observation period required to acquire the entire navigation message. For instance, the navigation message of the GPS signals only updates every 30 seconds and a GPS receiver must maintain a direct line-of-sight with a GPS satellite for at least 12.5 seconds to acquire the ephemeris information portion of the navigation message. For example, a user in a vehicle equipped with a GPS navigation system may wish to determine his position while traveling among high-rises in a city center. The only time the user's GPS receiver can come in contact with the GPS satellites is when the user's vehicle is crossing an intersection which is typically less than 12.5 seconds. Because the time a GPS receiver in a roving vehicle comes in contact with the GPS satellites is limited and often less than 12.5 seconds, the GPS receiver of a roving vehicle cannot acquire updated ephemeris information needed to accurately determine its current position. The same is true for a user carrying a GPS receiver while roaming inside a building where the GPS signals are either obstructed or weak at best. To operate his GPS positioning system, the user must remain in an open area for at least 12.5 seconds so that the receiver can obtain the ephemeris information necessary to calculate its position.
One proposed solution is to install stationary GPS receivers at each of the base stations of a cellular network. The base station GPS receivers receive and retain updated ephemeris information for the GPS satellites within its view. The updated ephemeris information is then transmitted to roving GPS receivers in the vicinity of the base stations. However, this implementation has several limitations. First, the roving GPS receivers must be physically close to a stationary GPS receiver to obtain the ephemeris information for the same set of GPS satellites in view. Thus, the range of the base station GPS receivers is limited. Second, the base stations are vulnerable to radio frequency interference or hardware failures. If only a single or a few base stations are deployed, interference or malfunctions at one base station can mean an overall system failure. Third, special software must be installed on each base station GPS receivers to facilitate transmission of ephemeris information, adding to the implementation expense.
Thus, it is desirable to provide a method for distributing GPS satellite information from GPS satellites to roving GPS receivers not able to remain in contact with GPS satellites for a sufficient amount of time to acquire the necessary information.